Original fix was wrong, this sorts it out I think. Delta values had to
be doubled as we're in Clip space (I think) and the matrix
multiplication order had to be swapped around.
Updated one of the signla/slot pairs to be simpler as we no longer need
to pass the zoom percentage back and forth.
The translation needs to be scaled relative to the zoom percentage so
we get sensible movement when dragging. Dragging is also disabled when
the image is smaller than the container widget.
Re-orders the initialisation of some variables to stop a compile
warning on Travis.
Thomas Wilshaw